It is well accepted that the treatment of any disease is easier and more effective if an underlying pathology is diagnosed as early as possible. For some diseases early diagnosis (preferably before the appearance of clear clinical symptoms) is critically important because of pathology transition into a more advanced, sometimes irreversible, stage. For example, one of the major problems for drug development and treatment of Alzheimer's Disease and other neurodegenerative diseases is their late clinical manifestation and diagnosis due to high compensatory potentials of the brain. As a result, these diseases are usually diagnosed when many neurons are already dead and currently the best case scenario is the prevention of pathology worsening, not a real recovery. Cancer is another example, since treatment of the metastatic stage of the disease is much more problematic than the treatment of the primary tumor. There are many other pathologies of this kind, but again the treatment of any disease is more effective if it is diagnosed earlier.
There are several basic types of clinical tests: (i) genetic tests that help to predict predisposition to a particular disease; (ii) screening tests, which are applied to a large population for early detection of a disease, preferably prior to its clinical manifestation; (iii) diagnostic tests, which are applied when a person has clinical symptoms of a disease or when the pathology has been detected by a screening test; (iv) molecular predictive tests that should predict the disease outcome and drug sensitivity.
Screening tests are most important for the early detection of a disease. It is true not only for spontaneous diseases but also for genetically linked pathologies, for which high chances of getting a disease are predicted by genetic testing. Ideally, everybody should undergo regular screening for all possible life threatening and many other diseases. There have been numerous attempts to develop tests for early detection of various diseases and different screening tests are currently performed for specific risk groups. For example, periodic colonoscopy is recommended to people over 50 years old, Pap smears are recommended to women and the PSA test to men for early detection of the cervical and prostate cancer, respectively. A main advantage of these tests is their disease-specificity, but at the same time it is their serious disadvantage because each test addresses only one particular pathology. However, there are many hundreds of human diseases, and it is difficult to imagine that such specific screening tests would be developed for all of those pathologies. Moreover, even if specific tests for early detection of all human diseases have been developed, it is highly unlikely that such tests would be used for screening purposes, especially for relatively rare diseases due to economic factors. Because screening tests for each particular disease address large populations their specificity and positive predictive value (PPV) are very important. For example, if a screening test for a relatively common disease (1:10,000) is 100% sensitive and 99% specific, which is almost impossible to achieve, and 1 million people are screened, 100 cases would be detected correctly but about 10,000 people would receive false positive results. Obviously, such outcome would cause emotional distress for these people as well as significant financial consequences for additional tests.